You might consider trying to reduce the number of unnecessary chemicals your daughter is exposed to in her hygeine routine. You can do this by buying natural shampoos/body cleansers like castile or 100% vegetable soaps. (I like Dr. Bronner's peppermint, lavender and hemp castile liquid soaps and also Trader Joe's Tea Tree Oil and Burt's Bees bar soaps.) These soaps clean skin and hair just as well as traditional soaps, but don't have all the harsh sulfates. Also, I'd cut out any kind of bubble baths or salts you may let her use and make her shower till it goes away. Then you can try to reindtroduce a more natural type of bath foam or salt. (Whole Foods has some good organic/no sulfate ones. I like to make my own soak from either baking soda or epsom salts and add a couple drops of different essential oils that smell nice. It's way cheaper and it's more healthy for you.)
You should also consider that her rash could be an allergic reaction to something in her diet. There are many potential allergens that regular pediatricians don't seem to have on their radars. I was seeing a holistic MD for some health issues I had recently and she suggested taking this test that samples blood and urine for over 90 possible food and enviromental allergens that can manifest strange symptoms in your body. Most traditional doctors don't look at food allergens as a factor in health issues, but they are apparently the cause for many problems. If you see a holistic doctor for your daughter's rash, make sure they're an MD, otherwise you can't be sure if you're getting knowledgeable advice.
Good luck and I hope that something works out for you and your daughter soon!